Motor vehicles today, e.g. cars, trucks and buses, are usually provided with cruise control. An object of cruise control is to achieve a uniform predetermined vehicle speed. This is done either by adjusting the engine torque to avoid retardation, or by applying brake action on downhill runs where the vehicle is accelerated by its own weight. A more general object of cruise control is to achieve convenient driving and greater comfort for the vehicle's driver.
A driver of a motor vehicle with cruise control usually chooses a set speed vset as the speed which he/she wishes the vehicle to maintain on level roads. A cruise control then provides an engine system of the vehicle with a reference speed vref which is used for controlling the engine. The set speed vset may therefore be regarded as an input signal to the cruise control, and the reference speed vref as an output signal from the cruise control, which is used for controlling the engine.
In today's traditional cruise controls (CC), the reference speed vref is identical with the set speed vset set by the user of the system, e.g. a driver of the vehicle. Today's traditional cruise controls therefore maintain a constant reference speed corresponding to the set speed vset set by the driver. The value of the reference speed vref is here altered only when the user him/herself adjusts it during the journey.
There are today cruise controls, so-called economical cruise controls, e.g. Ecocruise and similar cruise controls, which try to estimate current running resistance and also have knowledge of historical running resistance.
An experienced driver using a vehicle without cruise control can reduce fuel consumption by adapting his/her driving to the characteristics of the road ahead so that unnecessary braking and/or fuel-consuming acceleration can be avoided. A further development of these economical cruise controls tries to mimic the experienced driver's adaptive driving on the basis of knowledge of the road ahead, so that fuel consumption can be kept as low as possible, since this very greatly affects profitability for an owner of the vehicle, e.g. a haulage company or the like.
An example of such a further development of an economical cruise control is a “look ahead” cruise control (LACC), i.e. a strategic cruise control which uses knowledge of sections of road ahead, i.e. knowledge of the nature of the road in front, to determine the configuration of the reference speed vref. Here the reference speed vref is therefore allowed, within a speed range, to differ from the set speed vset chosen by the driver, in order to drive in a way which saves more fuel.
Knowledge of the road section ahead may for example comprise prevailing topology, road curvature, traffic situation, roadworks, traffic density and state of road. It may also comprise a speed limit for the road section ahead and a traffic sign beside the road. This knowledge may for example be obtained from location information, e.g. GPS (global positioning system) information, map information and/or topographical map information, weather reports, information communicated between different vehicles and information communicated by radio. These different types of knowledge may be used in various ways. For example, knowledge of a speed limit ahead may be used for fuel efficiency by lowering the vehicle's speed before the speed limit zone is reached. Similarly, knowledge of a road sign conveying information about, for example, a roundabout or intersection ahead may also be used for fuel efficiency by braking before reaching the roundabout or intersection. Basing cruise control on positioning information in combination with topographical map information makes it possible to avoid incorrect decisions due to a driver's misperception of a road gradient. For example, a driver may misinterpret the surroundings as indicating a downhill run ahead of the vehicle, whereas the topographical map data may directly and correctly indicate that what is actually there is an uphill run. When the vehicle reaches the section of road ahead, a good driver will appreciate that a correct decision was arrived at on the basis of the positioning information in combination with the map data. His/her acceptance of the function is thereby increased.
An economical cruise control may for example predict the vehicle's speed along a horizon of any suitable length, e.g. about 1-2 km. The vehicle's future speed along the horizon is predicted in various ways such as driving with traditional cruise control at a reference speed vref which is the same as the set speed vset, or varying the reference speed vref relative to the set speed vset.
A look-ahead cruise control (LACC) for example allows the reference speed vref to be raised, before a steep climb, to a level above the set speed vset, since the vehicle will presumably lose speed on the steep upgrade owing to high train weight relative to engine performance. Similarly, the LACC allows the reference speed vref to drop to a level below the set-speed vset before a steep downgrade, since on such a downhill run the vehicle will presumably be accelerated by its high train weight. The concept here is that it is better from a fuel economy perspective to take advantage of the vehicle's acceleration by its own weight downhill than to initially accelerate before the downgrade and then brake downhill. The LACC can thus reduce fuel consumption without journey time being affected.
There are also cruise controls which use a current running resistance as a basis for deciding how the vehicle's speed should vary. In such cruise controls, the reference speed vref can be allowed to deviate from the set speed vset on the basis of at least one characteristic of the running resistance, e.g. its magnitude and/or pattern over time.
In this specification, the invention is exemplified for use in, or in combination with, a cruise control system, e.g. a look-ahead cruise control (LACC), i.e. a strategic cruise control, which can use knowledge of the nature of the road ahead to control the reference speed vref. The invention may however be implemented in, or in combination with, substantially any cruise control in which the reference speed vref can be allowed to differ from the set speed vset.
As mentioned above, an LACC has knowledge of the vehicle's location and local topography, but other parameters may also affect the reference speed vref which a driver wishes the system to use as an output signal. Such parameters may comprise for example other road users and how they behave in traffic. Another such parameter might be a prevailing traffic situation, e.g. roadworks.